Bug 42597: Fix `#updatePubkeys`.

Bug 42597: Fix generateInvite.

We make sure to call handle_issue_invite to convert the response from
"issueinvite" into credentials.

We also stop stringifying the lox credentials, which are already a
string.

Bug 42597: Made sure we're always using `JSON.parse(request).request`
where `request` is generated by lox_wasm.

Bug 42597: Move duplicate logic into `#makeRequest`.

Moreover, we pass in the common fetch arguments into buildPostRequest
and treat the return value as a string instead of a JSON object.

Bug 42597: Refactor error handling.

A lot of the errors can be propagated to the caller.

Bug 42597: Make buildPostRequest accept fetch arguments.

This allows the request to buildPostRequest in the Lox module use the
same arguments that would be passed to `fetch`.

Also, return the text stream, rather than JSON.

Bug 42597: Add note for missing trusted invites.